This invention relates to a time code signal processing circuit.
ANSI/SMPTE standard 12M-1986 specifies a format and modulation method for a digital code to be recorded on a longitudinal track of video tape by a video tape recorder to facilitate timing and control in subsequent use of the recorded video material, for example in editing. The code, which is commonly known as SMPTE time code, specifies a unique and complete address on the video tape for each television frame. During playback of the video tape, the time code signal is read from the tape and is decoded.
The time code signal is a pulse signal, specifically a biphase signal. In biphase coding, a signal epoch is divided into time slots, and one source data bit occupies a single time slot. Each source data bit may be represented by a two-cell doublet. Each coding doublet begins, and therefore also ends, with a transition. A source data bit one generates a transition between the two cells of the doublet, whereas a source data bit zero does not.
Standard 12M-1986 prescribes that the transition time of the pulse signal, i.e. the rise time and the fall time, shall be 25.+-.5 .mu.s, corresponding to a slew rate from 41.67*V.sub.pp v/ms to 62.5*V.sub.pp v/ms, where V.sub.pp is the peak-to-peak voltage of the time code signal, and devices designed for utilizing the time code signal are generally designed around this range of slew rates.
A video tape recorder designed for professional use has a wide range of playback speeds typically ranging from jog (one frame per second) through normal (about 30 frames per second) to shuttle (64 frames per second). Since the time code signal is synchronized with the video signal, this implies that the slew rate of the time code pulses can vary between about one-thirtieth the slew rate at normal speed and about 64 times the slew rate at normal speed, and that the slew rate is linearly related to the time code signal's data rate. Furthermore, the amplitude of the time code signal provided by a conventional VTR may vary depending on playback speed, such that the amplitude at high speed is greater than at normal speed and the amplitude at low speed is less than at normal speed. The wide range of variation in slew rate and the variation in amplitude can lead to difficulties, particularly in clocked propagation of the time code signal. In particular, a routing switcher designed for switching a digital signal normally requires that the signal that is being switched have a fairly narrow range of slew rates and amplitudes. Therefore conventional switchers are not well suited to switching time code signals.